DESCRIPTION: The overall objectives of the proposed research are to understand the breadth and the mechanisms of a novel signaling pathway from mitochodria t the nucleus called retrograde regulation, whereby nuclear gene expression changes in response to alterations in the functional state of mitochondria. Th retrograde response pathway is known to require three genes, RTG1,RTG2, and RTG3. The following specific aims are proposed: 1) search for new targets of the RTG genes, and search for new retrograde responsive genes using promoter trap and RT-PCR screens; 2) analyze mechanisms of a transcriptional switch in which the RTG genes function as mitochondrial stress response genes in that they are recruited to the expression of some genes encoding mitochdrial proteins, primarily in cells with reduced or compromised mitochondrial activities; 3) test the details of a general model for control of the retrograde response pathway, which posits that the low abundance basic helix-loop-helix/leucine zipper (bHLH/Zip) transcription factor, Rtg3p, is sequestered in an inactive form by an inhibitor, which it is proposed is encoded by a fourth gene in the pathway, RTG4, in cells with fully functional mitochondria. In cells with dysfunctional mitochondria, the putative molecular chaperone activity of Rtg2 is effectively increased, releasing the sequestered Rtg3p to make it available to heterodimerize with the higher abundant bHLH/Zip transcription factor, Rtg1p, for transcriptional activation; and 4) to determine the role of the retrograde pathway in the 'artificial' apoptotic response in yeast involving Bax and Bcl2. Recent findings have revealed an intriguing relationship between mitochondrial function and programmed cell death (apoptosis) in multi-cellular organisms. Apoptosis is a process that plays an important role in development, tissue homeostasis and oncogenesis. There is now clear evidence that dysfunctional mitochondria can initiate a cascade of events leading to apoptosis involving the pro-apoptotic membrane pore forming protein, Bax, and its antagonist Bcl-2. When expressed in yeast, Bax has been reported to kill respiratory competent rho+ cells but only causes a growth arrest of respiratory incompetent rho o petites. Overall, these studies should provide new insights into how cells respond to changes in the functional state of mitochondria.